Modern surgical RF instruments of the bipolar type allow nowadays new methods as well as surgical operating techniques for a permanent and safe thermofusion of vessels and tissue structures. The high reliability achieved by these instruments in sealing processes offers a wide application spectrum also in the field of minimally invasive techniques with particularly short access paths, whereby a reduced time of operation and reduced costs of operation can be accomplished.
Thermofusion is essentially based on two mechanisms that must match with each other in the best possible way so as to obtain a good fusion result, viz.                firstly, on the application of current via two opposed instrument branches equipped with electrodes and used for fixing therebetween the tissue to be treated, and        secondly, on the contact pressure (clamping force) with which the tissue to be treated is compressed between the instrument branches.        
Both factors influence and physically change the collagen and elastin structures and produce also the parchmenty thermofusion zone which optically indicates safe sealing of the vessels.
The fundamental surgical course of action in executing thermofusion by means of a known type of RF sealing instrument can be described uniformly for all known instruments as follows:
The tissue to be severed and sealed is first grasped with the two branches of the instrument and clamped therebetween with a defined clamping force. Subsequently, the thermofusion mode is activated via a manual switch or a foot switch, whereupon the electrodes of one branch have applied thereto an RF current, which is conducted through the clamped tissue to the electrode of the opposite branch thus thermofusing the tissue. The supply of current is stopped either manually or automatically as soon as the optimum fusion quality has been accomplished. In view of the bipolar nature of the instrument, lateral thermal damage is avoided to a large extent. Finally, the tissue is cut through in the middle of the fusion zone by means of a (mechanical or electrical) scalpel or a sharp-edged forceps constituting a further component of the instrument.
In practice, RF sealing instruments of the bipolar type are successfully used, e.g. for vaginal hysterectomy or thyroidectomy executed by minimal incision techniques. However, also in other disciplines, such as enteroanastomosis, bipolar RF sealing instruments increasingly replace known standard instruments, such as the clip suture instrument which is also referred to as stapler. In this special case, the open surgical operation technique for executing an enteroanastomosis necessitates, among other prerequisites, the divisibility of the instrument into two instrument parts that are freely movable independently of one another. When an RF sealing instrument of the bipolar type is used, the connection of the tissue parts for anastomosing is therefore not accomplished by clips, but the tissue parts are so to speak “fused” by a high-frequency current. Nevertheless, the instrument must be divisible and the two instrument halves must be fully movable independently of one another.
The current must here flow from one instrument half including the electrodes incorporated in the branch via the clamped tissue to the counter electrode, which is located in the branch of the other instrument half. In order to allow sealing of different tissue thicknesses, contacting of the two instrument halves must moreover be possible even in the case of different electrode gaps of e.g. 0.1 mm to 1.5 mm.
For achieving an optimum sealing result under these boundary conditions, the target tissue must be present in a strongly compressed state in the area of the above-mentioned electrode gaps, and this can only be guaranteed when the instrument is fully closed via a closing lever that is specially provided for this purpose. Therefore, three positions of the sealing instrument that is adapted to be used for executing an enteroanastomosis are differentiated:                Open, and instrument halves (including branches) separated from one another: at this position both instrument halves can be moved fully freely in space independently of one another. In particular, the two instrument halves can, as is also necessary for executing an anastomosis, be introduced independently of one another in tissue volumes, such as in incisions made into the intestine.        Open, and instrument halves (including branches) connected to and articulated on one another, respectively: the two instrument halves are connected to one another via a plug connection/articulated connection at this position. The two instrument halves can therefore be moved about a common axis of rotation. The tissue can, at this position, still be displaced between the (spaced) branches. At this open position, an RF activation would therefore lead to an uncontrollable fusion result, since the contact pressure required for the fusion processes at the points of contact between the electrodes and the tissue does not exist. Hence, this is a fundamental safety risk that necessitates a safety concept excluding an activation at this position of the instrument.        Closed (tensioned): the instrument has been closed via the above-mentioned internal closing lever and the target tissue has had applied thereto a predefined (minimum) clamping force between the branches and has thus been compressed sufficiently. The target tissue can no longer be displaced between the branches at this position of the instrument. The position of the instrument relative to the tissue can only be changed after renewed opening. This is the only position at which activation of the RF current through the user is allowed.        
An RF bipolar sealing instrument of the type in question is known e.g. from U.S. Pat. No. 5,531,744. This instrument consists of two instrument halves which are articulated on one another and which each have provided thereon an electric connection for electrically connecting the respective instrument half to an RF generator. Each instrument half additionally comprises a handle as well as an instrument branch, respectively equipped with at least one electrode. Finally, a bar, which is adapted to be displaced in the longitudinal direction of the handle, is supported on the handle of one instrument half, said bar being coupled with a scalpel that can be extended from the handle along the instrument branch for severing tissue clamped between the instrument branches.
For clamping the tissue with a predetermined clamping force, one of the instrument halves has articulated thereon a closing lever, which has articulated thereon a pawl, as components of the respective handle, such that, when the two instrument halves are manually brought into contact with one another, the pawl can enter into locking engagement with a bolt on the opposite instrument half (opposite handle) so that, by operating the closing lever, a tension force can then be applied to the bolt and, consequently, to the instrument branches abutting on one another (with the tissue located therebetween).
Even if the RF (radio-/high-frequency) sealing instrument according to U.S. Pat. No. 5,531,744 should be demountable into its separate instrument halves, these two instrument halves remain, each via a power supply line of its own, constantly connected to the external RF generator, whereby the individual freedom of movement is limited also in the demounted condition of the instrument. In addition, this known instrument does not provide any protection preventing the instrument branches from having unintentionally applied thereto an RF current at a non-closed position of the instrument branches.